Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L5/00—Arrangements affording multiple use of the transmission path
  • H04L5/0001—Arrangements for dividing the transmission path
  • H04L5/0003—Two-dimensional division
  • H04L5/0005—Time-frequency
  • H04L5/0007—Time-frequency the frequencies being orthogonal, e.g. OFDM(A), DMT
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
  • H04L1/004—Arrangements for detecting or preventing errors in the information received by using forward error control
  • H04L1/0056—Systems characterized by the type of code used
  • H04L1/0071—Use of interleaving
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2626—Arrangements specific to the transmitter only

Definitions

• the invention relates generally to orthogonal frequency division multiplexing (OFDM) communication systems and, more particularly, to precoding techniques performed by such systems.
• OFDM orthogonal frequency division multiplexing
• the WiMedia standard defines the specifications of a media access control (MAC) layer and a physical (PHY) layer based on orthogonal frequency division multiplexing (OFDM) transmissions.
• the WiMedia standard enables short-range multimedia file transfers at rates up to 480Mbps with low power consumption.
• the standard operates in a frequency band between 3.1GHz and 10.6GHz in the UWB spectrum.
• FIG. 1 shows a block diagram of a multiband OFDM (MB-OFDM) transmitter 100 that operates in accordance with the WiMedia specification.
• the transmitter 100 includes a channel encoder 110, a bit interleaver 120, a symbol mapping unit 130, and an OFDM modulator 140.
• the transmitter 100 implements a bit-interleaved coded modulation (BICM) technique to overcome frequency selective fading of a UWB channel.
• BICM bit-interleaved coded modulation
• the channel encoder 110 encodes input information bits that are later interleaved at a bit level by the bit interleaver 120, and then mapped to symbols by the symbol mapping unit 130.
• BICM bit-interleaved coded modulation
• the bit interleaver 120 performs three steps: 1) inter- OFDM-symbol interleaving, where consecutive bits are distributed to different OFDM symbols, which may be transmitted in different sub-bands; 2) intra-symbol tone interleaving, where bits are permuted across the data sub-carriers of an OFDM symbol to exploit frequency diversity; and 3) intra-symbol cyclic shift interleaving, where bits are cyclically shifted in successive OFDM symbols to exploit more frequency diversity when only one sub-band is used.
• the OFDM symbols are generated and transmitted through a transmit antenna 150. These OFDM symbols are transmitted either in one sub-band (FFI mode) or multiple sub-bands through a frequency hopping, which is controlled by a time frequency code (TFC).
• TFC time frequency code
• WiMedia PHY transfer rate can be increased to 960Mbps if a 3 A convolutional code together with 16 QAM modulation are used.
• precoding of the transmitted OFDM symbols is required to ensure good performance.
• the precoding technique is needed to avoid loss of frequency diversity gain resulting from the characteristics of the OFDM transmission.
• the precoding techniques are based on jointly modulating transmit symbols onto multiple subcarriers. This allows a receiver to recover transmit symbols even when some of these subcarriers are in deep fading.
• Examples of the precoding techniques can be found in "OFDM or single-carrier block transmissions?,” by Z. Wang, X. Ma and G. B. Giannakis published in IEEE Transactions on Communications, vol. 52, pp. 380-394 March 2004 and in "Linearly Precoded or Coded OFDM against Wireless Channel Fades,” by Z. Wang, and G. B. Giannakis published in Third IEEE Signal Processing Workshop on Signal Processing Advances in Wireless Communications, Taoyuan, Taiwan, March 20-23, 2001.
• the precoding is typically performed by a precoder circuit coupled to an input of the OFDM modulator 140 of a transmitter.
• a well-designed complex precoder can effectively exploit the frequency diversity provided by multi-path channels.
• implementing a complex precoder increases the complexity of the transmitter and receiver, as it requires more sophisticated decoding and symbol mapping techniques.
• DCM dual carrier modulation
• a higher constellation e.g., 256QAM
• a more efficient precoding technique is multiple-subcarrier-joint-modulation (MSJM) precoding.
• MSJM precoding allows to jointly modulate multiple symbols onto multiple subcarriers while utilizing a minimum constellation size. Thus, implementing such a technique does not require designing complex and costly precoders.
• Certain embodiments of the invention include an orthogonal frequency division multiplexing (OFDM) multiband transmitter adapted to perform multiple-subcarrier-joint- modulation (MSJM) precoding.
• the transmitter comprises a bit interleaver for independently interleaving bit blocks and grouping bits of each interleaved bit block to bit groups; a symbol mapping unit for mapping each of the bit groups to symbols according to a MSJM scheme; and a symbol interleaver for allocating the symbols to data subcarriers of a plurality of consecutive OFDM symbols.
• Certain embodiments of the invention include a method for performing a multiple- subcarrier-joint-modulation (MSJM) precoding in a multiband frequency division multiplexing (OFDM) transmitter.
• the method comprises arranging encoded information bits into bit blocks; independently interleaving each bit block; grouping the interleaved bits of each bit block to bit groups; mapping each of the bit groups to symbols, wherein the number of symbols is determined according to a number of jointly modulated symbols; and allocating the symbols to data subcarriers of a plurality of consecutive OFDM symbols.
• Certain embodiments of the invention include a computer-readable medium having stored thereon computer executable code for performing a multiple-subcarrier-joint- modulation (MSJM) precoding in a multiband orthogonal frequency division multiplexing (OFDM) transmitter.
• the computer executable code causing a computer to perform the process of arranging encoded information bits into bit blocks; independently interleaving each bit block; grouping the interleaved bits of each bit block to bit groups; mapping each of the bit groups to symbols, wherein the number of symbols is determined according to a number of jointly modulated symbols; and allocating the symbols to data subcarriers of a plurality of consecutive OFDM symbols.
• Figure 1 is a block diagram of a multiband OFDM transmitter
• FIG. 2 is a block diagram of a multi-band OFDM transmitter implemented in accordance with certain embodiments of the invention.
• Figure 3 is a table showing exemplary values of the bit interleaver parameters
• Figure 4 is a diagram illustrating the interleaving of symbols
• Figure 5 is a table showing exemplary symbols sequences for different TFC codes.
• Figure 6 is a flowchart describing an exemplary method for implementing a MSJM precoding in a multiband OFDM transmitter realized in accordance with an embodiment of the invention.
• FIG. 2 shows a non-limiting and exemplary block diagram of a multiband OFDM transmitter 200 implemented in accordance with certain embodiments of the invention.
• the transmitter 200 is based on a standard multiband OFDM transmitter and is adapted to perform the MSJM precoding described above.
• the transmitter 200 may support data rates of lGbps and above and operate according to the specification of WiMedia 1.0, 1.5, and 2.0,
• WiMax WiMax
• IEEE 802. Hn IEEE 802. Hn standards.
• the transmitter 200 includes a channel encoder 210, a bit interleaver 220, a symbol mapping unit 230, a symbol interleaver 240, and an OFDM modulator 250.
• the channel encoder 210 and the OFDM modulator 250 have the same functionality as a standard channel encoder (e.g., encoder 110) and OFDM modulator (e.g., modulator 140) utilized in multiband OFDM transmitters.
• the symbol interleaver 240 is added to the design, and certain parameters of the bit interleaver 220 and symbol mapping unit 230 are reconfigured.
• bit interleaver parameters such as a TDS factor, a code bits/OFDM symbol (N CBPS ), a tone interleaver Block Size (NT ⁇ ne ), a cyclic interleaver shift (N cyc ) are set to different values.
• bit interleaver parameters such as a TDS factor, a code bits/OFDM symbol (N CBPS ), a tone interleaver Block Size (NT ⁇ ne ), a cyclic interleaver shift (N cyc ) are set to different values.
• a table showing exemplary values of such parameters for supporting data rates of 640Mbps, 800Mbps, and 960Mbps is provided in Figure 3.
• the 'I' interleaved bits of a bit block are divided into a number of 'p' bit groups, where each bit group includes 'k' bits.
• Different grouping schemes may be used for this purpose. Following are a few non-limiting examples for the grouping schemes. In these examples, the number of bit groups 'p' equals to 200 and the number of bits 'k' in each group is 12, the i-th bit group is represented as G(i) and b(j) is the j-th bit in a bit block.
• One possible grouping scheme is:
• bit interleaver 220 implements the third grouping scheme to achieve good performance.
• the symbol mapping unit 230 maps each bit group G(i) to a number of m symbols using the MSJM precoding scheme. In a preferred embodiment, the mapping is performed using a lookup table. When utilizing the lookup table the values of the bit groups are used as table indexes to retrieve the values of the symbols. Therefore, the mapping of bit groups to symbols includes a single lookup operation. It should be noted that both the transmitter 200 and a receiver are preconfigured with the lookup table for performing the mapping and de- mapping operations.
• the output of the symbol mapping unit 230 includes a number of m*p symbols (or a number of p m-symbol tuple [x(i,0), ..., x(i,m-l)]).
• the symbol interleaver 240 allocates the m*p symbols to a number of 'n' data subcarriers of 's' consecutive OFDM symbols.
• the number 's' is an integer number equals to m*p/s.
• the symbol interleaver 240 distributes the 'm' symbols in the same m-symbol tuple to different subcarriers of different OFDM symbols. This is further illustrated in Figure 4, where three OFDM symbols 410, 420, and 430 carry the symbols in the tuple [x(l), x(2), x(3)].
• the symbol x(l) is carried by subcarrier 33 in the OFDM symbol 410
• symbol x(2) is carried by subcarrier 66 in the OFDM symbol 420
• the symbol x(3) is carried by subcarrier 99 in the OFDM symbol 430.
• the OFDM symbols may be transmitted on different sub-bands, thereby maximizing the diversity gain that can be achieved.
• the symbol interleaving is performed using an n-modulo operation ('n' is the number of data subcarriers in an OFDM symbol), and the sequence to transmit the symbols is determined according to the TFC code.
• n' is the number of data subcarriers in an OFDM symbol
• the sequence to transmit the symbols is determined according to the TFC code.
• the number of symbols 'm' in a tuple is 3 and the number of data subcarriers 'n' is 100, the number of bit groups 'p' is 200, therefore the number of consecutive OFDM symbols 's' is 6, i.e., 3*200/100.
• d o (i) x(i,O)
• d 2 (i® k 2 ) x(i,2) and
• ⁇ / 5 ((/ - 100) e Ar 2 ) JC(/,2)
• ⁇ denotes a module 100 addition operation
• ki and k 2 are two positive integers with 0 ⁇ ki, k 2 ⁇ 100 and kl ⁇ k2.
• FIG. 5 shows a non-limiting flowchart 600 describing the method for implementing MSJM precoding in a multiband OFDM transmitter realized in accordance with an embodiment of the invention.
• encoded information bits are arranged into bit blocks, each of which includes a number of 'I' bits.
• the parameter 'I' equals to 2400 bits.
• each bit block is independently interleaved to output a number of 'I' interleaved bits.
• the interleave bits are grouped into a number of 'p' bit groups, each of which includes 'k' bits. In a preferred embodiment, 'p' equals to 200 groups and the value of 'k' is 12 bits.
• each bit group is mapped to a number of 'm' symbols. The mapping is performed using a lookup table in which the bit values of a bit group are used as the table indexes to retrieve the symbol values.
• these symbols are allocated to a number of 'n' data subcarriers of 's' consecutive OFDM symbols according to a predefined TFC code. The allocation is performed according to symbol intereleaving scheme described in detail above. In a preferred embodiment, the value of 'n' is 100, and 's' equals to 6.
• the OFDM symbols are transmitted in different sub-bands. In a FFI mode, the symbols are transmitted in a single sub-band.
• the principles of the invention are implemented as a combination of hardware, firmware and software.
• the software is preferably implemented as an application program tangibly embodied on a program storage unit or computer readable medium.
• the application program may be uploaded to, and executed by, a machine comprising any suitable architecture.
• the machine is implemented on a computer platform having hardware such as one or more central processing units (“CPU"), a memory, and input/output interfaces.
• CPU central processing units
• the computer platform may also include an operating system and microinstruction code.
• the various processes and functions described herein may be either part of the microinstruction code or part of the application program, or any combination thereof, which may be executed by a CPU, whether or not such computer or processor is explicitly shown.
• various other peripheral units may be connected to the computer platform such as an additional data storage unit and a printing unit.

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• Error Detection And Correction (AREA)
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• 2009
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